In cathode ray tubes, such as those used in color television sets or computer display monitors and the like, it is important that the electron beam used to excite, or illuminate, the CRT screen be properly aligned so as to strike or excite only the intended point or pixel (pixels). Where the electron beam fails to strike only the intended pixel or pixels, undesirable results such as improper color or general lack of image clarity, or sharpness, may result. Since electron beams used in common cathode ray tubes are sensitive to magnetic fields or magnetisms, a typical CRT such as that used in a color television set and the like, can be affected by external magnetic fields or magnetisms, such as terrestrial magnetism. The result is often poor image quality. More particularly, external magnetic fields, such as terrestrial magnetism can affect the electron beam tracking so as to cause the beam to misland, or not strike the desired target point (pixel or pixels). Where the electron beam strikes the desired target point, the electron beam is said to have made a perfect landing.
Generally, terrestrial magnetism causes a mislanding of an electron beam on a cathode ray tube. So a method for correcting a track of an electron beam which is used for a cathode ray tube has been required. Especially a method for correcting a track of an electron beam, of which a structure is not complicated, has been required to get a perfect landing of an electron beam on a cathode ray tube.
FIG. 2 is a view of a prior art which is disclosed in U.S. Pat. Nos. 3,571,652 and 5,148,083. In this case, a magnetism shielding board 1 is provided around a cathode ray tube. The magnetism shielding board 1 may be provided inside of a cathode or outside of a cathode ray tube.
FIG. 2 shows a case wherein the a direction of a terrestrial magnetism M.sub.1 is from left to right. Generally demagnetization coils are provided at an upper side and a lower side of a cathode ray tube. The coil provided at the upper side is electrically connected to the coil provided at the lower side. An alternating attenuated electric current is flown to each demagnetization coil. An alternating attenuated magnetic field m having a direction which is perpendicular to the direction of the alternating attenuated electric current is generated by the alternating attenuated electric current. The direction of the alternating attenuated magnetic field m is also perpendicular to the direction of the terrestrial magnetism M.sub.1. When the terrestrial magnetism M.sub.1 and the alternating attenuated magnetic field m works on the CRT, a reverse magnetic field M.sub.2 (a demagnetizing field) having a direction which is reverse, or opposite, to the direction of the terrestrial magnetism M.sub.1, is generated on the CRT. Because the shielding board 1 is a paramagnetism material such as a silicon steel plate, the magnetic shielding board 1 is magnetized in the direction of the reverse magnetic field M.sub.2 thereby.
The strength of the terrestrial magnetism M.sub.1 is expressed as H.sub.1. The strength of the reverse magnetic field M.sub.2 is expressed as H.sub.2. A strength H.sub.0 of a total magnetic field M.sub.0 is expressed as H.sub.0 =H.sub.1 -H.sub.2. The strength H.sub.0 of the total magnetic field M.sub.0 correlates to a mislanding of an election beam on a cathode ray tube.
In this case, the strength H.sub.2 of the reverse magnetic field M.sub.2 is smaller than the strength H.sub.1 of the terrestrial magnetism M.sub.1. A relation between the strength H.sub.1 and the strength H.sub.2 is determined by the magnetism shielding board 1. Namely, the relation is expressed by the following formula: EQU H.sub.2 =aH.sub.1
When the magnetism shielding board 1 is made of a silicon steel plate, the value of a is from 0.3 to 0.6.
Thus, because the strength H.sub.2 of the reverse magnetic field M.sub.2 is absolutely smaller than the strength H.sub.1 of the terrestrial magnetism M.sub.1, the strength H.sub.0 of the total magnetic field M.sub.0 never becomes zero. So a mislanding of an electron beam on a cathode ray tube strick occurs even though a demagnetization coil and a magnetic shield boards are provided in a CRT for correcting a mislanding of an electron beam.